The present invention relates in general to a system for controlling a hydraulic load, and in particular to a system which includes a source of pressure fluid, a pressure conduit connecting the source to the load and an electrohydraulically operated servo-control valve arranged in the pressure conduit.
A hydraulic control system of this type is known from the German published patent application No. 23 64 559 in which it is disclosed how an extrusion cylinder of a die casting machine for plastic materials is supplied with a pressure fluid via an electrohydraulic servo-control valve. Particularly in such control systems which are used in connection with die casting machines for plastic material it is necessary that during the filling phase of the process the servo-control valve adjust the system to feed through the extrusion cylinder a considerably increased amount of pressure fluid at a low pressure drop in the control valve whereas in the subsequent compression phase only small amounts of pressure fluid must be supplied. The servo-control valve permits that the pressure fluid flowing to the extrusion cylinder can be very accurately controlled in a broad range of its pressure and quantity. Nevertheless, the disadvantage of this known control system is the fact that the nominal flow rate of the servo-control valve has to be adjusted to the largest amount of the pressure fluid that still would cause a small permissible pressure drop across the valve. As a consequence, the prior art control system of this type are relatively expensive and as much they require large and expensive servo-control valves. In addition, when large servo-control valves are employed, the accuracy of the control of the pressure fluid decreases proportionately to the decrease of the pressure fluid flow.
From the U.S. Pat. No. 3,550,505 a device for controlling a hydraulic load is also known in which a throttle is connected in a pressure conduit between a pressure fluid source and a multiway control valve and the resulting pressure difference of the throttle controls an auxiliary control valve. The effect of this known combination of a throttle with an auxiliary valve and additional structural elements is limited to a device having a single pump and two separate load circuits whereby the purpose of this combination is to control the dual loads independently from each other. In addition, even in this known device the normal flow rate of the multiway control valve has to be adjusted to the maximum amount of the pressure fluid which has to be supplied to the load.